1. Field of the Invention
The present invention relates to dual mode piezoelectric filters and, more specifically, relates to a filter construction that improves the degree of coupling between the two modes and reduces losses within the passband.
2. Description of Related Art
Conventional filters used in mobile phones and other wireless communication devices include dielectric filters, laminated filters, acoustic wave filters, and the like. Among acoustic wave filters, well-known types include quartz crystal filters (MCF: monolithic crystal filters), which utilize multiple bulk wave modes, surface acoustic wave filters (SAW filters), etc. However, recent years have seen increased demand for miniaturization, upgraded performance characteristics, and higher frequencies, and piezoelectric filters (FBAR filters), which utilize bulk waves in piezoelectric thin films, have been developed as devices satisfying those requirements. Moreover, multiple-mode piezoelectric filters, which are implemented by multiplexing multiple modes, have been proposed in the field of piezoelectric filters.
A conventional configuration that utilizes multiple modes in the above-mentioned MCFs has been disclosed in JP H10-163804A. FIG. 18A is a schematic plan view illustrating the MCF configuration disclosed in JP H10-163804A, and FIG. 18B is a sectional view. The MCF has a dual-mode filter construction wherein a piezoelectric substrate 51 is used to form an integral vibratory portion 52, which is an ultra-thin strip, and a surrounding thick annular enclosure portion 53 supporting the vibratory portion 52, and, in addition, two electrodes 54a, 54b with a width of W and a length of L are arranged, with a gap “g” provided therebetween, on top of the piezoelectric substrate 51, and a full-surface electrode 55 adhered to the rear side. Lead electrodes 56 extend from the electrodes 54a, 54b toward the surrounding thick edge portion, with bonding pad electrodes 57 provided on the edge portion. In the recessed portion, the full-surface electrode 55 serves double duty as a lead electrode.
Acoustic resonance is generated, for instance, by using electrode 54a as an input electrode and electrode 54b as an output electrode and applying potentials between, respectively, the electrode 54a and the fill-surface electrode 55, and between the electrode 54b and the full-surface electrode 55. The filter is formed by coupling two modes, i.e. a primary mode and a secondary mode, which are generated as a result of the acoustic resonance. Moreover, if the central dimension of the vibratory portion is designated as A=(A1+A2)/2 and its dimension in a direction normal thereto is designated as B, then, in order to suppress the spurious effects generated when the electrode film thickness is reduced and the passband of the filter is expanded, A/(2W+g) is set to between 1.4 and 1.8 and the B/L ratio is set to a range of from 1.3 to 1.7.
Moreover, another conventional configuration utilizing multiple modes in the above-mentioned MCFs has been disclosed in JP 2005-269241A. FIG. 19 is a cross-sectional view showing the MCF configuration disclosed in JP 2005-269241A. This MCF is a dual mode filter formed by arranging first and second electrodes 62, 63 on one of the major surfaces of a piezoelectric substrate 61, with a predetermined gap provided therebetween, and, at the same time, placing a third and fourth electrodes 64, 65 on the other major surface opposite to the first and second electrodes 62, 63. Although in the conventional configurations the electrode on one of the major surfaces was a full-surface electrode, the CI value (equivalent resistance at resonance) of the symmetric mode deteriorated beyond that of the anti-resonance mode and the yield declined, as a result of which the deterioration had to be suppressed by adhering a metal film 66, which was sufficiently large in comparison with said two electrodes 62, 63, on top of the two electrodes 62, 63 on one of the major surfaces of the piezoelectric substrate 61.
Moreover, a configuration of a dual mode piezoelectric filter, in which aluminum nitride (AlN) was used as the piezoelectric film and the fundamental wave of thickness-longitudinal vibration was used as the predominant vibration, was disclosed in JP2004-147246A. FIG. 20 is a cross-sectional view illustrating the dual mode piezoelectric filter configuration disclosed in JP 2004-147246A. In this dual mode piezoelectric filter, a structure obtained by forming an input electrode 81 and an output electrode 82 on a piezoelectric plate 71 made of aluminum nitride is supported on a supporting substrate 86 made of silicon. A grounding electrode 83 and a nitride silicon film 84 are interposed between the supporting substrate 86 and the piezoelectric plate 71, with a silicon oxide film 85 formed on a major surface of the piezoelectric plate 71. This construction generates an s—0 mode, i.e. a symmetric mode, and an a—0 mode, i.e. a diagonal symmetric mode, thereby producing a band-pass filter.
Moreover, because this is a layered construction, in which dielectric films of approximately the same thickness are provided on both major surfaces of the piezoelectric plate 71, and because the resonant frequencies of the two vibration modes are stable characteristics, it can yield a filter characteristic with a stable bandwidth, etc.
Next, explanations are provided regarding the characteristics of an exemplary conventional dual mode piezoelectric filter, in which aluminum nitride (AlN) is used for the piezoelectric thin film and the fundamental wave of thickness-longitudinal vibration is used as the predominant vibration. FIG. 21(a) illustrates the construction of an exemplary conventional dual mode piezoelectric filter utilizing an AlN film and the fundamental wave of thickness-longitudinal vibration as the predominant vibration, and (b) illustrates the distribution of the two generated vibration modes (the symmetric mode and diagonal symmetric mode).
The dual mode piezoelectric filter 90 includes a bottom electrode 94 formed on the top face of the substrate 95, a piezoelectric thin film 91 made of AlN formed on the bottom electrode 94, and two top electrodes 92, 93 formed on the piezoelectric thin film 91. Furthermore, a cavity portion 96 is formed in the substrate 95 so as to cover the area of the two top electrodes 92, 93.
A first vibratory portion is composed of the top electrode 92, a portion of the bottom electrode 94 that lies opposite to the top electrode 92, and a portion of the piezoelectric thin film 91 interposed therebetween. A second vibratory portion is composed of the top electrode 93, a portion of the bottom electrode 94 that lies opposite to the top electrode 93, and a portion of the piezoelectric thin film 91 interposed therebetween. The two vibratory portions are separated by providing a gap between the respective top electrodes 92, 93. Furthermore, with the vibration in the two vibratory portions ensured by the presence of the common cavity portion, the construction generates two vibration modes, i.e. a symmetric mode and a diagonal symmetric mode, as shown in FIG. 21(b).
Furthermore, FIGS. 22A to 22C illustrate the results of analysis conducted in case of the exemplary conventional dual mode piezoelectric filter shown in FIG. 21. FIG. 22A, in which planar-direction wavenumber is plotted along the abscissa and frequency is plotted along the ordinate, shows wavenumber distributions in the planar direction at various frequencies. The center of the abscissa is 0, with the right side showing real wavenumbers and the left side showing imaginary wavenumbers. Since there is no propagation in the planar direction at the frequency at which the wavenumber is 0, it indicates the resonant frequency of thickness-longitudinal vibration. Moreover, the figure shows that there is active propagation in the planar direction at frequencies, at which the wavenumber is real, and propagation attenuates at frequencies, at which the wavenumber is imaginary. This curve is commonly called a dispersion curve.
Furthermore, the curve is said to be of the high-cut type when the wavenumber is imaginary in the high frequency area, and of the low-cut type when the wavenumber is imaginary in the low frequency area. Piezoelectric materials of the high-cut type include aluminum nitride (AlN), PZT, etc., and piezoelectric materials of the low-cut type include ZnO, quartz, etc. Moreover, the choice between the high-cut type and low-cut type can be controlled by changing the direction of polarization of the piezoelectric thin film. Furthermore, FIG. 22B, in which frequency is plotted along the abscissa and the imaginary part of admittance is plotted along the ordinate, illustrates the respective admittance characteristics T0, S0 of the symmetric mode and diagonal symmetric mode shown in FIG. 21. FIG. 22C, in which frequency is plotted along the abscissa and insertion losses are plotted along the ordinate, illustrates the filter characteristic. The insertion losses are shown such that the top edge is 0, with the losses increasing towards the bottom.
In FIG. 22A, C1 shows a dispersion curve of the first vibratory portion and second vibratory portion, and C2 shows a dispersion curve obtained for the piezoelectric thin film 91 and bottom electrode 94 alone, without the top electrodes 92, 93. Both C1 and C2 show high-cut type curves. Here, when thickness-longitudinal vibration is the predominant vibration, the resonant frequency depends on the thickness and density of each layer, with the resonant frequency shifting to a lower range when the product of thickness×density is larger. This phenomenon is commonly referred to as the “mass loading effect”, with C1 shifting to a lower range commensurately with the magnitude of the mass loading effects of the top electrodes 92, 93. At such time, the characteristic of C2 exhibits real wavenumbers in the vicinity of the resonant frequency of C1, i.e. in the vicinity of the frequency, at which the wavenumber in the planar direction reaches 0. Accordingly, vibration excited in the first vibratory portion or in the second vibratory portion actively propagates to areas free of the top electrodes and ends up leaking outside in the planar direction. As a result, as shown in FIG. 22B, the characteristics of the two vibration modes deteriorate, and, furthermore, as shown in FIG. 22C, the filter characteristic ends up exhibiting significant insertion losses.
Next, with reference to FIG. 23, explanations will be provided regarding another configuration and characteristics of an exemplary conventional dual mode piezoelectric filter obtained when aluminum nitride (AlN) is used for the piezoelectric thin film and the fundamental wave of thickness-longitudinal vibration is used as the predominant vibration. FIG. 23(a) shows the construction of the dual mode piezoelectric filter, and FIG. 23(b) shows the distribution of the two generated vibration modes (symmetric mode, diagonal symmetric mode). The piezoelectric filter has an energy confinement-type configuration, which provides particular improvements in terms of vibration energy leakage, such as in the case of the piezoelectric filter shown in FIG. 21.
The dual mode piezoelectric filter 100 has a bottom electrode 94 formed on one of the surfaces of the substrate 95, a piezoelectric thin film 91 of AlN formed on the bottom electrode 94, and two top electrodes 92, 93 formed on the piezoelectric thin film 91. A cavity portion 96 is formed in the substrate 95 so as to cover the area of the two top electrodes 92, 93. Furthermore, mass load elements 97a, 97b are provided on the outside and in the planar direction of the top electrodes 92, 93. It should be noted that, in this specification, the mass load elements are defined as elements producing the above-described mass loading effect.
Here, the density and the thickness of the mass load elements 97a, 97b is set such that (ρ1×h1)<(ρt×ht) and (ρ2×h2)<(ρt×ht), wherein h1 is the thickness and ρ1 is the density of the top electrode 92, h2 is the thickness and ρ2 is the density of the top electrode 93, and ht is the thickness and ρt is the density of the mass load elements 97a, 97b. 
A first vibratory portion is composed of the top electrode 92, a portion of the bottom electrode 94 that lies opposite to the top electrode 92, and a portion of the piezoelectric thin film 91 interposed therebetween, and a second vibratory portion is composed of the top electrode 93, a portion of the bottom electrode 94 that lies opposite to the top electrode 93, and a portion of the piezoelectric thin film 91 interposed therebetween. The two vibratory portions are separated by providing a gap between the respective top electrodes 92, 93. In addition, vibration in the two vibratory portions is ensured by the presence of the common cavity portion 96 and, furthermore, vibration energy is confined to the first vibratory portion and second vibratory portion by providing the mass load elements 97a, 97b, thereby generating the two vibration modes shown in FIG. 23, i.e. the symmetric mode and diagonal symmetric mode.
FIGS. 24A to 24C illustrate the results of analysis conducted for the exemplary conventional dual mode piezoelectric filter shown in FIG. 23. FIG. 24A, in the same manner as FIG. 22A, shows wavenumber distributions in the planar direction at various frequencies, wherein a wavenumber in the planar direction is plotted along the abscissa and frequency is plotted along the ordinate. In the same manner as FIG. 22B, FIG. 24B, in which frequency is plotted along the abscissa and the imaginary part of admittance is plotted along the ordinate, illustrates the respective admittance characteristics T1, S1 of the symmetric mode and diagonal symmetric mode shown in FIG. 23. In the same manner as FIG. 22C, FIG. 24C, in which frequency is plotted along the abscissa and insertion losses are plotted long the ordinate, shows the filter characteristic.
In FIG. 24A, C1 shows a dispersion curve of the first vibratory portion and second vibratory portion. C2 shows a dispersion curve obtained for the piezoelectric thin film 91 and bottom electrode 94 alone, without the top electrodes 92, 93, i.e. for the region between the top electrodes 92, 93. C3 shows a dispersion curve obtained for the regions, in which the mass load elements 97a, 97b are formed. C1, C2, and C3 all show high-cut type curves. Here, the increased density and thickness of the mass load elements 97a, 97b causes C3 to shift to a lower range in comparison with C1. In other words, since the characteristic of C3 exhibits imaginary wavenumbers in the vicinity of the resonant frequency of C1, the propagation of vibrations excited in the first vibratory portion or second vibratory portion attenuates in the region, in which the mass load elements 97a, 97b are formed, and the outward leakage in the planar direction is reduced. As a result, as shown in FIG. 24B, the characteristics of the two vibration modes are improved, and, furthermore, as shown in FIG. 24C, the filter characteristic is improved as well.
The construction illustrated in FIG. 23 reduces the specific vibration energy leakage that occurs when using high-cut type piezoelectric thin films and improves the filter characteristic. However, it is difficult to use because, as shown in FIG. 24C, the curve is not smooth within the passband. The reason is that in the construction of FIG. 23, the resonant frequency ftr of the symmetric mode T1 and the anti-resonant frequency fsa of the diagonal symmetric mode S1 are set far apart, as a result of which the sign becomes the same between these frequencies, while lossless passage is made possible by setting the admittances of the symmetric mode T1 and diagonal symmetric mode S1 shown in FIG. 24B so as to have opposite signs in the desired frequency band.